Light-weight, hand-held metal detectors are popularly used by hobbyists and collectors to search for buried or otherwise hidden metal objects of value or particular interest, such as coins, jewelry, and artifacts of historical significance. Such metal detectors typically employ a transmit coil through which current flows, the current establishing time-varying magnetic fields that induce eddy currents in metal objects, and interact with any magnetic permeability of the metal object. These effects are detected in a receive coil, and at least the eddy current effects are indicative of the metal object, permitting its identification.
In addition to metal objects of interest, the ground itself typically contains metallic compounds, particularly compounds containing iron. The iron compounds in particular have a relatively high magnetic permeability that often masks the response of the detector to the metal objects in the ground. It is a problem in the metal detector art to eliminate all the ferrous mineral signals in the target volume of ground while retaining sufficient information to identify the metal objects.
Two classes of hand-held metal detectors have been developed in response to this problem. “Induction balance” detectors produce a continuous periodic interrogating signal. A substantially steady state response due to induced eddy currents in the target is measured during interrogation and is characterized by a magnitude and phase. The effect of ground mineralization is eliminated by adjusting the phase of detection so that it is insensitive to the phase angle of the ground. However, while the magnitude of the response for objects whose phase response is off this angle is thereby detected, the phase response itself is not, so one metal object cannot be distinguished from another.
As a solution to that problem, induction balance metal detectors have used what is known as “motion filtering,” which eliminates the effect of ground mineralization by making use of the fact that the ground mineralization is typically distributed over a larger area and is therefore relatively constant in space as compared to the metal objects of interest, such as coins. In motion filtering, the detector is moved over the ground and the rate of change of the response is determined, with the lower frequencies corresponding to the slower changing affects of ground mineralization being filtered out. With the effects of ground mineralization eliminated by motion filtering, the phase angle of the response can be used to discern one metal object from another.
More recently, multiple frequency induction balance has been developed to eliminate the need for motion filtering. In this method, two or more discrete frequencies are used providing two or more ground balanced signals which can be compared to permit discrimination between targets as well as ground nulling. However, metal objects are not equally responsive to the different frequencies, so there is generally a single frequency that is optimum for discerning particular metal objects. Accordingly, a disadvantage of multiple frequency excitation is that the energy that would otherwise be used for the optimum single frequency is divided among several frequencies, with the result that the energy in each frequency is reduced and the depth of detection consequently suffers. This loss of energy efficiency can be a significant problem in battery powered metal detectors.
In induction balance detectors generally, a search head comprises at least two coils, one for transmitting and one for receiving. The coils are overlappingly arranged, or concentrically arranged along with a third, balancing coil, so that the effect of the transmit coil on the receive coil is nulled. Eddy currents induced in the target as a result of the target's response to the transmitted field, results in unbalancing the two coils, producing a response. The characteristic use of phase angles in induction balance detectors leads to this type being alternatively referred to in the art as “frequency domain” detectors.
The other of the two classes of metal detectors is known as “pulse induction,” in which a pulse is used to interrogate the target, and the target's response is measured after the interrogating pulse has ceased. To eliminate the major effect of ground mineralization, the method makes use of the fact that the response due to the permeability of the ground mineralization decays almost immediately. However, while the response due to magnetic permeability decays immediately, there is a remaining “magnetic remanence” or “viscous magnetic remanence” effect of the ground that, while not large, may remain in the response for some significant time after excitation. Moreover, the eddy currents induced in the target due to the presence of metal objects also decay after interrogation, and will decay quickly for small targets, so the signals are smaller than the signals obtained in an induction balance detector, making the pulse induction detectors generally less sensitive and less popular.
The use and analysis of a non-periodic time varying response in pulse induction detectors leads to this type also being referred to in the art as “time domain” detectors.
The pulse is critically damped with a damping resistor in order to ensure that the pulse stops as suddenly as possible without ringing, so that remaining eddy currents can be detected as soon as possible before they decay to such low levels that they are no longer useful. However, the damping resistor dissipates a significant amount of the energy used for interrogation. This energy loss is also particularly significant in a hand-held, battery-powered metal detector.
Barringer, U.S. Pat. No. 3,105,934 proposes an airborne electromagnetic system for the remote detection of ore bodies utilizing pulses of short duration to induce transient polarization in the ore bodies. From a loop transmitter in an airplane normally flying at a survey flying height of 500 feet, 80 half-sine current pulses per second are produced and are asserted to thereby radiate a primary, pulsed electromagnetic field. A “bird” having three receiving coils is towed by the airplane, the receiving coils being responsive respectively to three components of secondary fields resolved with respect to the direction of flight. It is proposed that an ore body consisting of base metal values occurring in a massive sulphide deposit may be considered as a conductive sheet in which circulating current will be induced in the presence of the primary field, and that the time constant for decay of this current, when the primary field is absent or not time varying, is indicative of the conductivity of the sheet, which aids in the recognition of varied ore deposits.
Excepting the use of half-sine pulses, for which no reason is given, the system employs the standard pulse induction methodology with its attendant drawbacks. Moreover, the system is not amenable to hand-held use or battery power.
Accordingly, there is a need for a pulse induction metal detector having high energy efficiency and sensitivity, particularly where the capability of the metal detector to reject ground mineralization and to discern and distinguish different metal objects in a target is not sacrificed.